Children. Children don't look. When I was very young I got stuck in a toilet in the middle of the night because my mom's boyfriend left the seat up. No one heard my cries for help. I was stuck for a while.

I did it one time when I woke up at 3am to pee. Had to keep the light off to not wake the Mrs.

It wasn't fun. But it wasn't life ending. I got a nightlight for the bathroom after that.

Habit and expectation can be powerful things. Also, our brains often fills in what we expect to see. A quick, peripheral glance at the toilet while expecting to see it in one state (seat down, lid up, for example) could leave one thinking they did see it in that state when in fact they didn't get a proper look at it.

When you wake up having to pee in the middle of the night and don't want to turn the light on

Also in general, as someone with a vulva, my mental model of a toilet is that the seat only is up for cleaning.

A blind person?

People that are used to living alone or with other women. Established habit and all that jazz.

I've always said if a person sits on a toilet with the seat up it's a failure on two parts.

Yes the previous user should've put the seat down but who tf just plops ass first onto something without looking first?

At that point you get what you deserve, a wet stinky ass.